Much of the recent research with Josephson junctions and their circuits, including that involving superconducting quantum interference devices (SQUIDs), has focused on the digital applications of such devices, primarily because of their extremely fast switching ability.
As discussed for example in U.S. Pat. No. 4,371,796 (Takada), such digital devices rely on the ability of a SQUID to quickly switch between a zero voltage state and a finite-voltage state due to changes in the magnetic flux through the loop of the SQUID, or by passing current exceeding the Josephson junction critical current through the SQUID, causing it to switch quickly between its two voltage states.
Although research has been ongoing for a number of years concerning the use of Josephson junctions for linear or analog amplification, it has proved to be difficult to produce a general purpose amplifier, which, in the present context, is defined to be an amplifier with the following properties: (a) apart from the signal which is to be amplified, it requires a source of power only at dc; (b) it must amplify all signals in a frequency range from zero up to some cut-off frequency; and (c) it must have an input impedance that is in the technically desirable range of one ohm or greater.
An example of previous work in this area is described in an early publication on Josephson junction amplifiers by J. Clarke, et al.: "Josephson-Junction Amplifier", Applied Physics Letters, Vol. 19, page 469 (1971). Clarke, et al. describe an inductively coupled asymmetric SQUID amplifier. Apart from an added resistance R.sub.i, the input impedance of this amplifier is very low, much less than an ohm and in essence an electrical short at low frequencies. Such low input impedance is common to practical Josephson junction amplifiers with an input that is inductively coupled, but it is technically undesirable for many applications.
Another early proposed amplifier configuration is discussed by Likharev in "The Properties of a Josephson Triode", Radio Engineering and Electronic Physics, Vol. 20, page 135 (1975). This amplifier employs a triangular configuration of three Josephson junctions. Several modes of operation are proposed but the detailed technical properties of this proposed amplifier have not been evaluated either theoretically or experimentally in the published literature.
A superconducting parametric amplifier based on Josephson junctions is discussed by Parrish, et al., in "Four Photon Parametric Amplification", Revue de Physique Appliquee, Vol. 9, page 229 (1974). This amplifier, later known as the SUPARAMP (see, for example, Feldman, "The thermally Saturated SUPARAMP", Journal of Applied Physics, Vol. 48, page 1301 (1977)) comprises a series array of Josephson junctions. It amplifies only at radio or microwave frequencies and requires a microwave frequency pump.
A SQUID amplifier is discussed by Zimmerman, et al., in "High Frequency Limitations of the Double-Junction SQUID Amplifier", Applied Physics Letters, Vol. 31, page 360 (1977). This amplifier, which is discussed as a radio or microwave frequency amplifier, could be used at frequencies extending down to dc, but, due to the inductive coupling of the input, its effective input impedance at low frequencies would be much less than one ohm. In this respect it is similar to the amplifier of Clarke, et al., discussed above.
A theoretical amplifier design is described by T. D. Clarke, et al., in "Feasibility of Hybrid Josephson Field Effect Transistors", Journal of Applied Physics, Vol. 51, page 2736 (1980). Such a transistor comprises a coplanar superconductor-semiconductor-superconductor Josephson junction with an insulated gate formed over the semiconducting channel. However, the recommended semiconductor material is difficult to fabricate and experimental results are not available to confirm that the design is practical.
Yet another Josephson junction amplifier is discussed by Van Zeghbroeck in "Superconducting Current Injection Transistor", Applied Physics Letters, Vol. 42, page 736 (1983). This amplifier consists of a large area Josephson junction in which the spatial distribution of the current is varied by a magnetic field produced by a control current. This device is capable of amplification over a wide frequency range but it has an input impedance so low that it is effectively a short at low frequencies.
A recently developed SQUID amplifier is discussed by Hilbert, et al., in "Radio-Frequency Amplifier Based on a dc Superconducting Quantum Interfernce Device", Applied Physics Letters, Vol. 43, page 694 (1983). Although only the radio frequency properties of the amplifier are discussed, the same design could be used at low frequencies. However, since this amplifier has an inductively coupled input its input impedance will be very low, essentially an electrical short at low frequencies, like the Clarke, et al. and Zimmerman, et al. amplifiers discussed above.
Another form of superconducting amplifier is discussed by Gray in "A Superconducting Transistor", Applied Physics Letters, Vol. 32, page 392 (1978), and by Faris, et al., in "QUITERON", IEEE Transactions on Magnetics, Vol. MAG-19, page 1293 (1983). These devices may have a relatively high input impedance and a large bandwidth but the mechanism for amplification relies on nonequilibrium quasiparticle tunneling, while an array amplifier constructed in accordance with the present invention is based on the electromagnetic phase locking of the Josephson oscillations, as will be discussed in more detail below.